Hundreds -- and perhaps more than 1,000 -- Soviet soldiers and Afghan civilians died in a major tunnel disaster in the Hindukush mountains in northern Afghanistan last week, according to reports reaching here through Western diplomatic channels and from Afghan exiles.

The reports said that a civilian fuel tanker truck collided with the lead vehicle of a Soviet Army convoy in the Salang Tunnel 60 miles north of Kabul on the main route linking the capital with the Soviet Union. The incident was said to have occurred Tuesday or Wednesday.

There was no official confirmation of the disaster from the Soviet military command or the Soviet-supported Afghan government.

There were conflicting and sketchy unofficial accounts of the disaster, but Western diplomatic sources today said that Soviet security forces, apparently believing a guerrilla attack had taken place, blocked both entrances to the 1.7-mile-long tunnel and prevented vehicles from leaving.

In the ensuing panic, 700 Soviet soldiers and 400 Afghan civilians suffocated from a lack of oxygen caused by the explosion and subsequent fire, according to the diplomatic reports.

As usually occurs with casualty reports from remote reaches of Afghanistan, there were conflicting accounts of the number of deaths and injuries in the reported disaster, and the incident was not even mentioned in one briefing conducted today by a Western diplomatic mission.

But diplomatic sources in another Western embassy, citing reports from Kabul, said the most common casualty figure cited there was 700 Soviet soldiers killed and another 200 injured.